Data network technology permits multiple users to share economically access to files in a number of file servers. Problems arise, however, in the assignment of files to particular servers. For example, it may be desirable to move a file from one file server to another when a new server is added to the network. A “CDMS” brand of data migration service provided by EMC Corporation of Hopkinton, Mass., can be used to move files from one file server to another while permitting concurrent client access to the files.
Files are also often moved between file servers in order to relocate infrequently accessed files from feature-rich, expensive, and highly-protected high-speed disk storage to more economical and possibly slower mass storage. In such a system, the high-speed disk storage is referred to as primary storage, and the mass storage is referred to as secondary storage. When a client needs read-write access to a file in the secondary storage, the file typically is moved back to the primary storage, and then accessed in the primary storage. This kind of migration of files between levels of storage in response to client requests based on file attributes such as the time of last file access and last file modification is known generally as policy-based file migration or more specifically as hierarchical storage management (HSM). It is desired for such policy-based or hierarchical storage management to be transparent to the clients, yet in practice there is always a trade-off between cost of storage and delay in file access.
In a data processing network employing hierarchical storage management, a client typically accesses a primary file server containing the primary storage, and the secondary storage is often in another file server, referred to as a secondary file server. When a file is moved from a primary file server to a secondary file server, the file in the primary file server is typically replaced with a stub file that contains attributes of the file and a link to the new file location in the secondary file server. The stub file can be accessed to redirect an access request from a client to the new file location in the secondary server, or to migrate data from the present file location back to the primary file server. This stub file can be a symbolic link file in a UNIX-based file system, or a shortcut file in a Microsoft WINDOWS file system. In a computer using the Microsoft WINDOWS operating system, access to a stub file may automatically result in access to the new file location. For example, an attempt to execute or open a shortcut will cause the Microsoft WINDOWS operating system to execute or open the target of the shortcut.
EMC Corporation has used a write policy known as “write recall full” for hierarchical storage systems employing its “CELERRA” (Trademark) brand of cached disk array for primary storage. In accordance with this “write recall full” policy, when a client requests the primary cached disk array for access to a file that has been migrated to secondary storage, either part or all of the “offline” file is copied from the secondary storage back to the primary storage. Once the copying brings part of the file “online,” the primary cached disk array may provide high-speed read-write access to the file. When the cached disk array is used in combination with much slower secondary storage, the “write recall full” policy may provide efficient and economical use of the storage and network resources.